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A 220-million-year-old fossil found in southwestern China appears to have settled a long-simmering debate over reptile evolution: how did turtles get their shell?

In a study published in the journal Nature, scientists report on the discovery of a missing-link species named Odontochelys semitestacea - meaning "toothed, half-shell turtle".

The scientists conclude that the outer shell of turtles emerges directly from the ribs and backbone and not from the skin.

The find also suggests that turtles originated in water rather than on land, and pushes back the group's first known appearance on earth by some 10 million years, to 220 million years ago.

Competing hypotheses

Since the era of dinosaurs, which roamed the planet until 65 million years ago, turtles have looked pretty much the way they do today.

They sport an armour-like upper shell, known as a carapace, connected to a softer lower part, called a plastron.

But in the absence of hard evidence, scientists argued over exactly how this reptilian mobile home came into being.

One school of thought said the shell evolved from skin.

According to this hypothesis, small bony plates called osteoderms - like those found on crocodiles - broadened in size to form a kind of dermal plating that fused over time with the ribs.

The competing hypothesis said that the plastron formed first, followed by an outgrowth and widening of the ribs and backbone to form the hard-shell carapace into which turtles withdraw to escape predators.

A similar process unfolds in the transformation of modern-day turtle embryos into hatchlings.

Belly first

"With Odontochelys, we now have clear fossil evidence of this process emerging in an adult," says Dr Xiao-chun Wu, a palaeontologist at the Canadian Museum of Nature in Ottawa and co-author of the study.

The team of scientists, led by Lau Li-Jun of the Zhejiang Museum of Natural History in Hangzhou, China also suggest that the new find points to an aquatic origin for turtles.

The fact that O. semitestacea has only a half-shell on top, but a fully-formed plastron is evidence that its underside was exposed to predators in the water.

"Reptiles living on the land have their bellies close to the ground with little exposure to danger," says co-author Dr Olivier Rieppel of The Field Museum in Chicago.

The scientists also found other marine reptiles and invertebrates embedded in the same rocks in Guizhou Province that yielded the new turtle species.